1. Field of the Invention
This invention relates to a process for producing a substrate for selective crystal growth, a process for selective crystal growth and a process for producing a solar battery by using such processes. Particularly, it relates to a process which can continuously perform selective crystal growth at low cost, a process for producing a substrate for selective crystal growth therefor and a process capable of continuously producing a solar battery with good energy conversion efficiency by use of the above selective crystal growth process.
2. Related Background Art
Solar batteries have been utilized as driving energy sources in various instruments.
A solar battery has a functional portion in which a pn junction or a pin junction is employed. Silicon has been generally employed as the semiconductor constituting these pn and pin junctions. With respect to efficiency in converting photoenergy to electromotive force, it is preferable to use single crystal silicon, but with respect to enlargement of area and reduction in cost, amorphous silicon has been deemed to be advantageous.
In recent years, for the purpose of low cost comparable with amorphous silicon and high energy conversion efficiency comparable with single crystal silicon, investigations have been made about use of a polycrystalline silicon. However, according to the method proposed in the prior art, a polycrystal shaped in a mass was sliced into a plate and used, and therefore it was difficult to make the thickness 0.3 mm or less, whereby the thickness cannot be made to a thickness thin enough to permit sufficient absorption of light possible. Thus, effective utilization of the material has not been possible. In short, for enhancing efficiency and reducing production cost, it is necessary to significantly reduce the thickness.
Accordingly, attempts have been made to form a thin film of a polycrystalline silicon by utilizing the thin film formation techniques such as chemical vapor deposition (CVD), etc., but the crystal grain size obtained by such method is at most a few microns, and the energy conversion efficiency is even lower when compared with the chip sliced from a mass of polycrystalline silicon.
Also, an attempt has been made to enlarge the crystal grain size by irradiating a polycrystalline silicon thin film, formed by the above-mentioned CVD method, with a laser beam, thereby effecting melting and recrystallizing the thin film. However, cost reduction was not satisfactory and production was unstable.
Such situation is not limited to silicon, but is the same with compound semiconductors.
On the other hand, as a process aiming at improvement of bulk productivity of a solar battery, there is the process disclosed in U.S. Pat. No. 4,400,409. This process comprises feeding a flexible substrate wound up on a pay out reel from the pay out reel, then conveying the substrate to a film forming chamber, performing film formation treatment in the film forming chamber and then winding up the film on a take up reel. Further, U.S. Pat. No. 4,400,409 discloses lamination of semiconductor layers different from each other by providing a plural number of film forming chambers. However, even when a solar battery was produced by this process, a polycrystalline film with a large crystal grain size as described above was not obtained, and satisfactory conversion efficiency was not obtained.
On the other hand, as a process for producing a solar battery of thin type, having sufficiently large crystal grain size and good energy conversion efficiency, there is the process disclosed in EP Laid-open Patent Application No. 276961A2. This application discloses "A process for preparing a solar battery comprising: effecting formation of a first semiconductor layer of a first conduction type on the surface of a substrate including the step of adding to the surface of said substrate a material different than the material constituting the surface of said substrate which is sufficiently greater in nucleation density (ND) than the material constituting the surface of said substrate in a sufficiently small area such that crystal growing occurs from only a single nucleus to thereby form a nucleation surface, applying crystal forming treatment to said substrate to form a single nucleus on the nucleation surface and growing a single crystal from the single nucleus; and then forming a second semiconductor layer of a second conduction type above said first semiconductor layer."
This process utilizes the selective single crystal growth method. The selective single crystal growth method is a method in which a crystal is permitted to grow selectively by utilizing the difference in factors between the materials which influence nucleation during a thin film formation process, such as surface energy, attachment coefficient, release coefficient, surface diffusion speed, etc. More specifically, it is a method in which a single crystal is permitted to grow on the base of a nucleation surface provided on a nonnucleation surface (surface with smaller nucleation density) having a sufficiently larger nucleation density than said nonnucleation surface and having a sufficiently fine surface area so as to form only a nucleus from which a single crystal is grown. In this method no crystal growth occurs from the nonnucleation surface, but growth of a single crystal occurs only from the nucleation surface. However, even if this method is utilized, there remain points to be improved for producing a solar battery of large area with good production efficiency.
In the process disclosed in the above-mentioned EP Laid-open Patent Application No. 276961A2, conventional patterning by using a generally known photolithographic method of the prior art is practiced in providing a different kind of material which becomes the nucleation surface on the substrate surface.
Such photolithographic step is done by batch treatment. For this reason, for obtaining a solar battery of high performance by use of a large gain size crystal, a plurality of steps were performed concurrently and continuously, whereby it has been difficult to improve production efficiency.
Thus, by use of any one of the processes as described above, it is extremely difficult under the present situation to produce a solar battery by use of a large grain size crystal capable of giving good conversion efficiency with good production efficiency.